Thermocouples are used to measure temperature and are often used in harsh environments such as gas turbine engines. Thermocouples typically comprise two wires of different metal, joined at their ends to form a loop. A temperature difference between the joined ends causes a current to flow around the loop, or a potential difference to be created. The difference in temperature between the two ends (the hot and cold ends) can be determined by measuring the potential difference set up when the circuit is open. If the temperature of the cold end is known, then the temperature of the hot end can be determined.
Several thermocouple units can be joined together to provide an average temperature measurement. In a gas turbine engine, for example, there may be eight or more thermocouple units coupled together in parallel.
A thermocouple unit consists of one, two or more thermocouple elements. The measuring end of each thermocouple is placed at the location at which temperature is to be measured while the other end (typically the cold end) is placed inside a thermocouple head and connected to a measuring circuit.
In order to provide improved system accuracy, an additional resistor, typically made from thermocouple material can be coupled to each thermocouple. This ensures that the thermocouple unit has a much higher resistance value when compared to the thermocouple harness that is used to connect thermocouple units together. This added resistor, called a ballast resistor, is manufactured using thermocouple alloys to ensure that the functionality of the thermocouple is not compromised.
FIG. 1 is a cross-section of a typical thermocouple unit 10 comprising a thermocouple head 12 in which a ballast resistor 14, is included. The thermocouple unit comprises thermocouple elements 16 connected to a ballast resistor 14 and to external circuitry through an output 18. A filling medium 20 is provided inside the thermocouple head to ensure that the wires within the thermocouple head are not damaged by vibration.
FIG. 2 is a circuit diagram of a typical K Type thermocouple, showing the ballast resistor 14 connected to one leg of the thermocouple 16. The K Type thermocouple shown in FIG. 2 comprises an NiAl leg 22 and a NiCr leg 24, with the ballast resistor 14 connected to the NiCr leg. The ballast resistor 14 is typically in the form of a sleeved wire that is wound into a coil, as shown in FIG. 1. Typically the resistor has a resistance of between 7 and 14Ω.
However, in order to improve the thermal efficiency of gas turbine engines, the operating temperatures of gas turbines are increasing. This increase in temperature means that the sensors in the engine have to survive in more extreme temperature conditions. Previous designs of thermocouple units, of the type illustrated in FIG. 1, have been designed to work with a head operating temperature below 400° C. However there is now a requirement to provide a thermocouple unit that is able to operate with head temperatures in excess of 470° C. It is an object of the present invention to provide a resistor and thermocouple unit that is able to operate at high temperatures.
EP2023106A discloses a thermocouple head unit including a pair of thermocouples in which the thermocouples are matched to have the same resistance. One of the thermocouples may be made longer, and coiled to fit within the casing, in order to match the resistance of the other. The thermocouples are mineral insulated and protected by a metal sheath.
Mineral insulated wire has also been used to form resistance heaters. For example, GB1518833A, U.S. Pat. No. 3,934,333 and FR2252674A disclose heater elements formed from mineral insulated wires.